Mechanized separation and recovery system for solid waste

ABSTRACT

Methods and systems for separating a mixed waste into a wet organic fraction, a dry organic fraction, and an inorganic fraction are achieved by (i) comminuting the mixed solid waste, (ii) fractionating the comminuted stream by size to produce two or more particle-sized waste streams, and (iii) processing one or more of the particle-sized waste streams using density separation to produce an intermediate wastes stream, and (iv) recovering a homogenous product (e.g., a recyclable material or an organic fuel) from the intermediate waste stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No.12/897,996 filed Oct. 5, 2010, titled “MECHANIZED SEPARATION ANDRECOVERY SYSTEM FOR SOLID WASTE”, which is a non-provisional of U.S.Provisional Patent Application No. 61/291,177 filed Dec. 30, 2009,titled “MECHANIZED WET AND DRY ORGANICS SEPARATION AND RECOVERY SYSTEMFOR MUNICIPAL SOLID WASTE”, both of which are hereby incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to systems and methods adapted for use inwaste recycling and conversion. More specifically, the present inventionrelates to the recycling and conversion of solid waste derived, forexample, from domestic and commercial refuse.

2. The Relevant Technology

Commercial, industrial, and residential consumers generate large amountsof throw-away and waste products (i.e., municipal solid waste) that needto be handled and disposed of in an environmentally satisfactory manner.Disposal of municipal solid waste (hereinafter “MSW”) has traditionallypresented a number of problems.

MSW is typically disposed of by landfilling and/or incineration. Thewaste products in landfills are typically either raw garbage orincinerator ash. These methods of waste product disposed contaminate thesoil, water and air. Environmental restrictions as well as land usagedemands for housing have reduced the number of sites available forlandfills.

Incineration (i.e., the mass burning of waste products) is rapidlybecoming a non-viable alternative in heavily populated areas. Publicoutcry over the air pollution associated with burning garbage has haltednearly all new incinerator construction.

In response to these waste disposal problems, governments and the publichave demanded that, wherever possible, recycling systems should beemployed both to conserve material resources and to reduce pollutionproblems. Efforts have been made to recover valuable resources such asglass, plastic, paper, aluminum, and ferrous and non-ferrous metals fromwaste materials. For example, households in many cities are asked tosort their garbage into recyclables (e.g., paper, plastic containers,metal containers and glass containers) and non-recyclables. However,rates of non-compliance and mis-compliance are high. That is, somecustomers fail to sort their waste or they sort incorrectly, whicheither shunts recoverable materials into the waste stream orcontaminates the recyclable stream with waste materials. Non-complianceand mis-compliance reduce the efficiency of and increase the costsassociated with operating a recycling system that is designed toprocessed pre-sorted waste.

In contrast, some recycling systems attempt to dispense with theproblems associated with pre-sorting by attempting to recover recyclablematerials from mixed waste. However, many of these systems are fraughtwith the tendency to be highly labor intensive to operate, whileoffering relatively low recovery rates of recyclables. Furthermore,these types of recovery facilities or programs still do not recover muchof the energy rich wet and dry organic materials, which are mostlylandfilled as residue.

Many recycling systems configured to work with pre-sorted waste or mixedwaste are designed to recover specific materials and/or form specificproducts. The components of these systems are arranged and designed torecover certain individual fractions such as combustible organicmaterials, aluminum, ferrous metals, glass, plastic, and miscellaneousbulky inorganic materials. Efficient resource recovery depends in largepart upon separating the maximum amount of desirable material from therefuse using relatively few separating components and minimizing thepercentage of unwanted materials in the individual fractions.

Nevertheless, the energy balance of many recycling systems is sub-paror, in some cases, negative. For example, many recycling systems tend tomisdirect recoverable materials, which reduces the efficiency ofdownstream processes and wastes much of the valuable energy and/ormaterials that may otherwise have been recaptured from the recoveredwaste. In other cases, the processes of recovering, transporting, andrecycling the recyclable materials are so inefficient that they consumemore energy than could be saved by simply landfilling the garbage andmaking new products from raw materials.

BRIEF SUMMARY OF THE INVENTION

Example embodiments of the present invention relate to methods andsystems for separating solid waste materials. More particularly,embodiments of the present invention relate to methods and systems forprocessing of mixed solid waste such as MSW into a wet organic fraction,a dry organic fraction, and an inorganic fraction. Efficient separationand recovery is achieved by (i) comminuting the mixed solid waste, (ii)fractionating the comminuted stream by size to produce two or moreparticle-sized waste streams, (iii) processing one or more of theparticle-sized waste streams using density separation to produce anintermediate waste stream, and (iv) recovering a one or more productsfrom the intermediate waste stream. The steps of comminution, sizeseparation, and density separation allow efficient separation andrecovery of wet organics, dry organics, and inorganic material fromhighly variable and highly diverse solid waste streams.

The foregoing processing steps generate intermediate waste streams thatare more easily and efficiently sorted using automated sortingequipment. For example, metals can be pulled from an intermediate wastestream because the sizes of the different materials are relatively closeand/or because materials of concentrations of same-type materialsincrease in downstream processes. The approach used in the methods andsystems of the present invention often avoids the common situation thatoccurs in prior art systems where, for example, a large piece of onematerial (e.g., a large piece of cardboard) covers a small piece of adifferent material (e.g., copper) and hinders proper separation andrecovery. The systems and method of the invention can recover largepercentages of materials that exist in low or high concentrationconcentrations in mixed waste streams. This allows the economicalrecovery of recyclable materials that were previously cost prohibitiveto recover using traditional techniques.

In various embodiments of the invention, the separated wet organics, dryorganics, and/or inorganic products can be separately processed toproduce valuable products. For example, the wet organic fuel product canbe digested to produce biogas and a compost material or dried andutilized as fuel. The dry organic fuel product can be used to produceenergy in various thermal conversion processes and/or be used as acarbon fuel substitute. Recyclable materials including glass, ferrousmetals, aluminum, non-ferrous metals such as brass, copper, stainlesssteel, other non-ferrous metals, and construction materials such asrock, stone, soil, etc. can be recovered from the inorganic fractionwhile the remaining inorganic debris or residue can be landfilled.Recyclable materials including cardboard and various rigid plastics suchas PETE and HDPE can be recovered from the dry organic stream.

In one embodiment, a method for separating solid waste is described. Themethod includes providing a solid waste stream that includes wet organicmaterial, dry organic material, and inorganic debris and comminuting(e.g., grinding or shredding) the solid waste to produce a comminutedwaste having a first size distribution. The comminuted solid waste isfractionated by size to produce an under fraction enriched in wetorganic material and an over fraction enriched in dry organic material.The over fraction and/or under fraction are further separated to producean intermediate waste stream from which wet organic products, dryorganic products, and/or inorganic products may be recovered. Theproducts may be homogenous products, which are therefore valuable as arecyclable (e.g., homogeneous metals or plastics) or as a usable product(e.g. a dry organic fuel).

Recovery of the product from the intermediate waste stream may beaccomplished by simply diverting a product to a bin (e.g., conveying awet organic product or a dry organic fuel to a storage facility) or itmay include the use of a single stream sorting apparatus. The use ofcomminution, size separation, and density separation can prepareintermediate waste streams suitable for being sorted using equipmentconfigured for single stream waste processing. For example, where ametal is recovered, the separation apparatus may be a magneticseparation unit such as a drum magnet, a cross-belt magnet, a headpulley magnet, etc.), to recover ferrous metal fractions; an eddycurrent separation unit to recover a non-ferrous metal fraction; and/ora Camera Optical Sorting Machine(s) and/or metal detector and/or othersuitable device to recover copper, brass and/or stainless steelnon-ferrous metal or a ferrous and non-ferrous metal. In someembodiments, undesired products can be pulled from the intermediatestream using a sorting apparatus to produce the desired product. Forexample, polyvinyl chloride (PVC) may be removed from a dry organicstream and/or a recyclable plastic stream can be recovered from a dryorganic stream using a Near Infrared Optical Sorting Machine and/orcamera optical sorter or other suitable device.

The methods and systems of the invention provide the ability to producemultiple intermediate waste streams from highly variable mixed waste,which allows valuable recyclables and fuel products to be economicallyrecovered and used. Moreover, the system may be used on high volumewaste streams such as MSW.

These and other objects and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only illustrated embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 is a flow diagram illustrating a method for separating solidwaste, according to one embodiment of the present invention;

FIG. 2 is a flow diagram illustrating a method for separating solidwaste, according to another embodiment of the present invention;

FIG. 3 is a flow diagram illustrating a method for separating solidwaste, according to yet another embodiment of the present invention;

FIG. 4 is a flow diagram illustrating a system for separating solidwaste, according to one embodiment of the present invention;

FIG. 5 illustrates a cut-away view of an air drum separator adapted foruse in the system for separating solid waste, according to oneembodiment of the present invention;

FIG. 6 illustrates a cut-away view of a gravity/air separation unitadapted for use in the system for separating solid waste, according toone embodiment of the present invention;

FIG. 7 illustrates a cut-away view of a hammer mill separator adaptedfor use in the system for separating solid waste, according to oneembodiment of the present invention.

FIG. 8 is a flow diagram illustrating a method for separating solidwaste, according to yet another embodiment of the present invention; and

FIG. 9 is a flow diagram illustrating a method for separating solidwaste, according to yet another embodiment of the present invention.

DETAILED DESCRIPTION I. Introduction and Definitions

Municipal Solid Waste (“MSW”) (i.e., trash or garbage) is a type ofwaste material that includes predominantly household waste withsometimes the addition of commercial wastes collected by a municipalityor a contractor hired by a municipality or by commercial businesseswithin a given area. They are in either solid or semisolid form andgenerally exclude industrial hazardous wastes.

MSW contains a number of components that have energy value, if properlyseparated, segregated and processed. For example, MSW may contain largeamounts of organic waste materials (e.g., food and kitchen waste, greenwaste (i.e., yard clippings, plants, vegetation, branches, lawn, etc.),paper, textiles, rubber, plastics and wood), recyclable materials (e.g.,cardboard and certain paper products, glass, ferrous metals, aluminumand other non-ferrous metals, and certain plastics), and inorganicwastes (e.g., concrete, dirt, rocks, debris). Rather than lumpingtogether essentially all MSW classes and either landfilling orincinerating it, it is preferable to separate any and all valuable orusable waste fractions and recover either the raw materials or theenergy content therefrom and landfill only those components that aretruly classified as refuse, which have no commercial or energy value.

In the present disclosure, a number of comminuting and/or sizefractionation steps are described with respect to methods and systemsfor the separation of solid waste. Typically each of these steps has anassociated size cut-off. Persons having skill in the art will appreciatethat fractionated materials typically exhibit a distribution ofparticles. The distribution will often include an insignificant numberof particles above or below the cut-off. As used herein, an uppercut-off number (e.g., 16″ or less, 12″ or less, 8″ or less, the upperrange of an 8″ to 2″ over fraction) generally means that about 90% ofthe particles in the fraction (i.e., the distribution) have a size ofless than the cut-off number, while about 10% of the particles in thefraction will be larger than the upper cut-off size. As used herein, alower cut-off number (e.g., the lower range of an 8″ to 2″ overfraction) generally means that about 90% of the particles in thefraction have a size of larger than the cut-off number, while about 10%of the particles in the fraction are smaller than the lower cut-offsize. In more preferred embodiments, upper cut-off number can include95%, even more preferably 99% of the of the particles in the fractionand/or the bottom cut can include less than 5%, even more preferablyless than 1% of the particles in the fraction.

II. Methods for Separating Municipal Solid Waste

Referring now to FIG. 1, a flow diagram illustrating the method 100 forseparating municipal solid waste is shown. In one embodiment, the methodincludes providing a mixed waste stream 102. The mixed waste stream mayinclude wet organic waste, dry organic waste, and inorganic waste. Inone embodiment, the weight percentage of wet organic waste, dry organicwaste, and inorganic waste in the mixed waste stream 102 is each(independent of one another) at least 5%, at least 10%, at least 20%, atleast 50%, or at least 75% (the sum of the three weight percentages notexceeding 100%). In one embodiment, the mixed solid municipal waste 102may be an unprocessed municipal waste. For example, solid waste stream102 may be provided directly from a municipal garbage collectionprocess.

Alternatively, solid municipal waste 102 may be partially pre-processed(e.g., by home owners) to remove a portion of the recyclable and/orrecoverable materials. For example, solid municipal waste 102 may bederived from a comprehensive residential or commercial waste stream thatcontains the remnant materials that exclude source separated materialscollected through recycling programs in which a portion of certainrecyclables (e.g., newspaper, cardboard, plastics, metal and/or glasscontainers) have been removed (i.e., the MSW may be a post-recycledwaste).

In either case (i.e. methods using unprocessed MSW or source separatedMSW), the mixed waste 102 may be manually pre-sorted to recover andremove items that are difficult to grind, obviously hazardous, and/orthat are particularly large (i.e., easily separated) and have a highrecovery value. The presorting may be performed by a loading operatingloading waste into the system or may be carried out by personnel on adedicated presorting line. For example, waste 102 may be metered onto apresorting conveyor where manual labor identifies items to bepre-sorted. Typically presorted items will include items that coulddamage or cause excessive wear to the grinder. Examples includeautomobile engine blocks, structural steel, tire rims, propane tanks,concrete blocks, large rocks, and the like. Hazardous waste ispreferably removed before grinding to avoid contamination with othermaterials in the mixed waste. Examples of obviously hazardous wasteinclude containers of solvents and chemicals, paint cans, batteries, andthe like. Presorting can also be used to recover particularly large andvaluable items that are easily picked from the mixed waste stream.Typically the recyclables recovered in the pre-sorting will be itemsthat are several times larger than the burden depth of the processstream such that they are easily visible and efficiently removedmanually. For example large cardboard boxes and metal pieces (e.g.,corrugated containers) can be recovered in presorting.

The mixed municipal solid waste is conveyed to a comminuting device suchas grinder or shredder (step 104). In one aspect, the conveyor in step104 may include a metering system such as a metering wheel configuredfor controlling the flow and associated burden depth of MSW such that arelatively constant amount of material is fed to the grinder or shredderover time (and optionally a pre-sort conveyor).

In step 106, the mixed waste is comminuted. The grinder used tocomminute the mixed waste stream may include one or more shafts thatinclude a number of cutting heads that that can cut and/or shredincoming waste materials to a selected size. Wastes ground by thegrinder will have a range of particle sizes. For example, some objectslike shipping pallets or tires will be ground or shredded, but mostparticles will be relatively large. In contrast, materials like glass,which tends to shatter, and food waste, which tends to shred, will bequite small.

As waste materials are ground or shredded by turning rotors mounted withcutting blades or knives against a rigid blade housing, they then dropthrough the grinder or shredder to the screen basket (circular punchplate or finned design screens). Materials having a ground size lessthan a selected size, such as about 16 inches or less, about 14 inchesor less, about 12 inches or less, about 10 inches less, or preferablyabout 8 inches or less then drop through a screen and move onto the nextstep in the process. Objects that are too large to pass through thescreen are typically recirculated repeatedly through the grinder orshredder until they are ground to a size that can pass through thescreen.

The ground or shredded solid waste can then be conveyed (step 108) to asize separator 110 that fractionates the ground or shredded municipalsolid waste by size to produce an under fraction 112 enriched in wetorganic material and an over fraction 114 enriched in dry organicmaterial. Suitable examples of a size separator 110 that can be used inthe present method include a disc screen separator, a trommel screenseparator, a vibratory screen separator and/or other size separatorsknown in the art.

Preferably, the comminuted waste from grinder 106 is ground or shreddedto a size of about 16 inches and below, and the over fraction has a sizedistribution with a ratio of small particles to large particles of lessthan about 8. More preferably, the comminuted waste is ground orshredded to a size of about 12 inches and below, and the over fractionhas a size distribution with a ratio of small particles to largeparticles of less than about 6. Most preferably, the comminuted waste isground or shredded to a size of about 8 inches and below, and the overfraction has a second size distribution with a ratio of small particlesto large particles of about 4 or less. Preferably, the under fraction112 has a size of less than about 2 inches.

According to the present method, one or both of the under fraction 112and the over fraction 114 may be further fractionated 116 and 118 bydensity and size to produce an intermediate stream from which valuableproducts may be recovered. The comminuting, size separation, and/ordensity separation may be used to produce any number of a wet organicproduct 120, a dry organic product 122, and/or and inorganic product124. In a preferred embodiment, the product(s) recovered from theintermediate streams is a homogeneous stream. Homogeneous streamsinclude recyclables and fuels that are sufficiently free fromcontamination to be recycled or used without further separation fromother types of components present in the mixed waste. Homogenousproducts can include single materials (e.g., aluminum) or a homogenousproduct can include a mixture of materials that impart a necessarytrait. For example, a homogenous dry organic fuel includes organicshaving a particular maximum water content and particle size and aminimum BTU value. The purity needed to achieve homogeneity will dependon the particular product. Those skilled in the art are familiar withpurities required for recycling of recovered waste materials and theproperties needed for organic fuels to be useful.

In one embodiment, either 116 or 118 may include one or more of (a)processing the under fraction 112 using a sizing screen to remove finesand then a first density separation unit to produce a heavy fraction anda light fraction, (b) processing the light fraction from the firstdensity separation unit using a second density separation unit toproduce a dry organic product and a wet organic product, (c) processingthe over fraction 114 using a third density separation unit to produce aheavy fraction and a dry organic product, and/or (d) processing theheavy fraction from the third density separation using a fourth densityseparation unit to produce a heavy fraction and a wet organic product.The density separation steps can employ an air separator, such as an airdrum separator or a gravity/air separator, a hammer mill separator, orother density separators known in the art, or a combination of these.The size or dimensional separators for fines can include disc screens,trommel screens, vibratory screen separators or other dimensionalseparators known in the art.

Product recovery (i.e., recovery of recyclables, wet organics, or dryorganics) can be achieved using any sorting equipment suitable for theparticular type of material being recovered. In aspect, the method mayfurther include using a magnetic separation unit (e.g., drum magnets,cross-belt magnets, head pulley magnets, etc.), to recover ferrous metalfractions; a eddy current separation unit(s) to recover a non-ferrousmetal fraction; or a Camera Sorting Machine(s) and/or metal detectors orother devices known to recover copper, brass and/or stainless steelnon-ferrous metal or a ferrous and non-ferrous metal fraction from oneor more of the under fraction, the over fraction, or the downstreamportion of one of these fractions, such as, but not limited to the heavyfraction or the light fraction from the first density separation unit,the dry organic product or the wet organic product from the seconddensity separation unit, the heavy fraction or the dry organic productfrom the third density separation unit, or the heavy fraction or the wetorganic product from the fourth density separation unit.

In one embodiment, the wet organic products recovered in the separationmethod can be further processed using a microbial digestion system toproduce biogas that can be used as fuel and compost that can be used asa soil amendment or dried to make an organic fuel for combustion as acarbon fuel substitute or thermal conversion into energy. Discussion anddescription of exemplary microbial digestion systems that can be used todigest the wet organic waste product produced in the current method canbe found in U.S. Pat. No. 7,615,155 entitled “Methods for removal ofnon-digestible matter from an upflow anaerobic digester,” U.S. Pat. No.7,452,467 entitled “Induced sludge bed anaerobic reactor,” U.S. Pat. No.7,290,669 entitled “Upflow bioreactor having a septum and an auger anddrive assembly,” and U.S. Pat. No. 6,911,149 entitled “Induced sludgebed anaerobic reactor,” and in U.S. Pat. Pub. No. 2008/0169231 entitled“Upflow bioreactor with septum and pressure release mechanism,” theentireties of which are incorporated herein by reference.

In one embodiment, the separated dry organic materials can be furtherprocessed to produce a dry organic fuel using one or more processingmethods known in the art. In one embodiment, the separated dry organicmaterial can be further comminuted to produce a desired particle sizefor combustion in a furnace. In one embodiment, a separated dry organicmaterial with an upper particle size in a range from 8-16 inches can becomminuted to produce a dry organic material with an upper particle sizeless than 6 inches, less than 4 inches, less than 2 inches, or less than1 inch.

The dry organic material may also be compacted, baled, re-shredded,pelletized or otherwise densified to facilitate transportation and/orhandling of the dry organic material in a combustion or conversionprocess. Densification can be carried out by baling, pellitizing orother compaction technique that provides a similar function. Preferablythe densification uses compaction rather than pelletization andpreferably produces a bale rather than a pellet. In one embodiment, thecompaction produces a unit of fuel with at least one dimension greaterthan 4 inches. The compacted fuel may be at least about 4, 6, or 12inches in diameter. Bales of compacted dry organic material may be oneto several feet in diameter.

The dry organic material may also be stored on-site in either a bulkstorage building with an automated filling and discharge system orstorage silos with unloading devices.

In one embodiment, the density of the compacted material may be in arange from about 2-60 lbs per cubic foot, preferably 3-30 lbs per cubicfoot, and most preferably 4-10 lbs per cubic foot.

The solid dry organic fuel can, for example, be used alone or withanother fuel in place of coal and other carbon based fuels in a numberof industrial and energy generation processes. The solid dry organicfuel can also be used as a fuel to make synthesis gas through a varietyof emerging high temperature thermal conversion processes (e.g.gasification, plasma arc gasification and pyrolysis). The solid dryorganic fuel can also be combusted in a solid fuel boiler or gasifierboiler to produce steam, which can be used to turn a steam turbine toproduce electricity.

While it may be desirable to recover value from essentially all thecomponents of a solid waste stream, the present invention includesembodiments in which all or a portion of the wet organic fraction, dryorganic fraction, or inorganic fraction is not fully separated into arecovered product. For example, in one embodiment the wet organicfraction and a portion of the inorganic fraction (e.g. glass) may remainmixed and simply land filled.

The present invention is particularly advantageous for recovering themajority of a particular type of material that is present in the mixedwaste stream in very low concentrations. The systems and methods allowprocessing of mixed waste stream to metaphorically speaking “pick theneedle out of the haystack.” In one embodiment, the mixed waste streammay include at least one type of recoverable material at a concentrationless than 15%, less than 10%, less than 5%, or even less than 1%, wherethe system or method is configured to recover at least 50%, at least70%, at least 80%, or even at least 90% of the particular recoverablematerial.

Referring now to FIG. 2, a method 200 for separating municipal solidwaste to produce a wet organic product, a dry organic product, and aninorganic fraction therefrom is illustrated. The method includesproviding a mixed municipal solid waste 202 that includes a wet organicfraction, a dry organic fraction, and an inorganic fraction. The mixedmunicipal solid waste may optionally be presorted as described abovewith reference to FIG. 1. In step 204, the mixed waste is conveyed to asolid waste grinder or shredder 206. Optionally the comminuted solidwaste can be passed under a suspended magnet (not shown) to recoverferrous metal, and the remaining comminuted waste 208 is then conveyedto a first size separator 210. First size separator 210 separates thecomminuted waste based on size to produce an under fraction 212 and anover fraction 236.

The method 200 further includes conveying the under fraction 212 fromthe first size separation unit 210 to a first density separation unit222. The first density separation unit 222 can be an air drum separatorthat uses air flow and a rotating drum to separate materials by density.Other air separation devices that function similarly can be used and areknown in the art. The first density separation unit separates the underfraction from the size separator 210 to produce a heavy fraction 218comprising primarily inorganic material and a light fraction 224comprising a mixture of dry organics and wet organics. The lightfraction 224 can be processed in subsequent density separation steps.

In an optional step, the under fraction 212 from the first sizeseparation unit 210 can instead be conveyed to an optional second sizeseparation unit 214 that is configured to produce an under fraction 216and an over fraction 220. The under fraction 216 is a fine fraction(e.g., less than ⅜ inch) and is primarily a wet organic product with asmall percentage of inorganic material. The coarse fraction 220 isconveyed to density separator 222 and treated similar to fraction 212 asdescribed above.

Examples of suitable size separators 214 that can be used in the presentmethod include a disc screen separator, a trommel screen separator,vibratory screen separator and/or other size separators known in theart. In one embodiment, the method 200 may include an optional step ofprocessing the wet organic product 218 from the second size separationunit 214 to remove at least a portion of residual inorganic debristherefrom. In one embodiment, the residual inorganic debris includesbroken glass.

In one embodiment, the second over fraction 220 has a size distributionwith a ratio of small particles to large particles of less than about 5and the second under fraction 216 has an upper particle size of lessthan about ⅕ of the large particle size of the over fraction 220. Forexample, the over fraction 220 from the second size separation unit 214may have a size distribution in a range from about 2″ to about ⅜″ andthe under fraction 216 has an upper particle size of less than about ⅜″.

Returning to light fraction 224, the light fraction 224 from the firstdensity separation unit 222 can be conveyed to a second densityseparation unit 226. The second density separation unit can be agravity/air separation unit that uses air flow to separate materials bydensity. The second density separation unit produces or separates aheavy fraction 228 wet organic material and a light fraction 232 dryorganic material. In one aspect, the heavy fraction 228 is a wet organicproduct and the light fraction 232 is a dry organic product 234. The wetorganic product 230 may be combined with wet organic product 262 forfurther processing and/or process separately and/or used in the same ordistinct processes (e.g., land filled as residue or used to generate abiofuel in an anaerobic digester.

Returning now to the over fraction 236, the over fraction from the firstsize separation unit 210 is conveyed to a third density separation unit238. The third density separation unit 238 can be an air drum separatorthat uses air flow and a rotating drum to separate materials by density.The third density separation 238 unit separates the over fraction fromthe size separator 210 to produce a light fraction 240 and a heavyfraction 244. The light fraction 240 is primarily a dry organic product242.

The heavy fraction 244 from the third density separation unit 238 isconveyed to a fourth density separation unit 246. The fourth densityseparation unit 246 can be a gravity/air separation unit that uses airflow to separate materials by density or it can be an air drum separatorthat uses air flow and a rotating drum to separate materials by density.Other devices that can be used for forth density separator include ahammermill separator (e.g., a Scott Turbo Separator) or a disc screen orvibrating screen or a combination of these. The fourth densityseparation unit produces a light fraction 248 and a heavy 252. In oneaspect, the heavy light fraction 248 is a wet organic product 250 andthe heavy fraction includes an inorganic fraction 254.

The density separator units (e.g., first, second, third, or fourthdensity separation units) may be calibrated to provide separationbetween wet organics, dry organics, and inorganic materials. In mixedmunicipal waste streams, these three different materials often exhibitdensities within particular ranges. For example, dry organics tend tohave a density of less than about 10 or 12 lbs/cubic foot; wet organicstend to have a density greater than 8, 10, or 12 lbs/cubic foot and lessthan about 60, 80, or 100 lbs/cubic foot; inorganic materials tend tohave a density greater than about 80 or 100 lbs/cubic foot. Thus, bysetting the density separators accordingly, the wet organic, dryorganic, and inorganic fractions may be separated based on density.Comminuting the mixed MSW prior to density separation increases theseparation efficiencies of the density separators.

In one embodiment, the method 200 may include a number of optional stepsusing metals separator systems 256, such as magnets or eddy currentseparators or camera optical sorting machines or metal detectors orother devices know in the art, to recover one or more of a ferrous metalproduct 258 or non-ferrous metal products from one or more of thefractions produced in the method 200. The method of claim 37, furthercomprising processing the under fraction from the first size separationunit using a magnetic separation unit to recover a ferrous metalfraction therefrom.

For example, the metal separation units 256 may be used to recover anyferrous and non-ferrous metal fractions from one or more of underfraction 212, the over fraction 236, the heavy fraction 218 or the lightfraction 224 from the first density separation unit 222, the dry organicproduct 234 or the wet organic product 230 from the second densityseparation unit 226, the heavy fraction 244 or the dry organic product242 from the third density separation unit 238, or the inorganicfraction 254 or the wet organic product 250 from the fourth densityseparation unit 246.

Referring now to FIG. 3, a method 300 for separating municipal solidwaste to produce a wet organic product, a dry organic product, and aninorganic debris residue product is illustrated. The method 300includes, providing an initial municipal solid waste stream 302 thatincludes wet organic waste, dry organic waste, and inorganic waste, andconveying 304 the waste 302 to a solid waste grinder or shredder 306.The initial municipal solid waste 302 is ground or shredded to produce aground or shredded waste 308 having a distribution of particle sizes.The ground or shredded waste 308 is then conveyed to a first sizeseparation unit 310 to produce a first under fraction 312 enriched inwet organic material and a first over fraction 346 enriched in dryorganic material. According the present invention, each of the underfraction 312 and the over fraction 346 may be further processed bydensity, and to a lesser extent, by additional size separation toseparate the under and over fractions 312 and 346 into wet organicproducts, dry organic products, and inorganic debris products.

In one aspect, the method 300 further includes processing the firstunder fraction 312 from the first size separation unit 310 using anoptional first magnetic separation unit 314 to recover a ferrous metalfraction therefrom, and further processing the first under fraction 312from the first size separation unit 310 using a second size separationunit 316 to produce a second under fraction 318 comprising wet organicproduct 324 and a residual inorganic debris and a second over fraction326 comprising dry organic material, wet organic material, and inorganicdebris. Optionally, the method 300 may include processing the secondunder fraction 318 with a glass removal apparatus 320 to remove at leasta broken glass portion therefrom and produce the wet organic product324.

In one aspect, the method 300 further includes processing the secondover fraction 326 from the second size separation unit 316 using a firstdensity separation unit 328 to produce a light fraction 330 and a heavyfraction 334, wherein the light fraction 330 comprises a residualinorganic debris 332, and wherein the heavy fraction 334 comprises wetorganic material and dry organic material. The heavy fraction 334 fromthe first density separation unit 328 may be further processed using asecond density separation unit 336 to produce a light fraction 342 and aheavy fraction 338. The light fraction comprises a first dry organicproduct 344. The heavy fraction 338 comprises a wet organic product 340.

Returning to the output from the first size separation unit 310, theover fraction 346 from the first size separation unit 310 may beprocessed using a third density separation unit 354 to produce a heavyfraction 356 and a light fraction 370. Optionally, the over fraction 346from the first size separation unit 310 may be processed with anoptional metal separation unit 350 positioned prior to the third densityseparation unit 354 to remove a ferrous or non-ferrous metal fractionfrom the over fraction from the first size separation unit 310. Ineither case, the heavy fraction 356 includes wet organic material,ferrous metals, and inorganic debris. The light fraction 370 includesdry organic material, ferrous metals, and non-ferrous metals.

In one aspect, the method 300 further includes processing the heavyfraction 356 from the third density separation unit 354 using a fourthdensity separation unit 358 to produce a heavy fraction 360 and a lightfraction 364. The heavy fraction 360 includes a third wet organicproduct 362. The light fraction 364 comprises an inorganic product 366that includes ferrous metals, non-ferrous metals, and inorganic debris.Optionally, the method 300 may include processing the light fraction 364from the fourth density separation unit 358 using a second magneticseparation unit 368 to recover a ferrous metal fraction therefrom.

Returning to the output from the light fraction 370 from the thirddensity separation unit 354, the method 300 further includes processingthe light fraction 370 from the third density separation unit 354 usingan optional PVC separator 384 to produce a product 370 that includes adry organic product 372. Optionally, the light fraction from the thirddensity separation unit can be conveyed 374 to a third magneticseparation unit 376 to recover a ferrous metal fraction therefrom. Theresidual fraction 378 can be processed using an eddy current separationunit 380 to recover a non-ferrous metal fraction therefrom. The residualfraction 382 from the eddy current separator 380 can then be processedusing an optional PVC separator 384 to produce a product 370 thatincludes a dry organic product 372. The optional PVC separator(s) canalso be programmed to recover certain recyclable plastics such as PETEand HDPE.

The wet organic products 386 can be further processed using one or moreanaerobic digesters to produce a biogas that can be used as a fuel and acompost that can be used as a soil amendment or dried to make an organicfuel for combustion as a carbon fuel substitute or thermal conversioninto energy. The dry organic fuel products 388 can, for example, be usedalone or with another fuel in place of coal and other carbon based fuelsin a number of industrial and energy generation processes. The dryorganic fuel can also be used as a fuel to make synthesis gas through avariety of emerging high temperature thermal conversion processes (e.g.gasification, plasma arc gasification and pyrolysis). Recyclable metalsand glass can be recovered from the inorganic fractions 390 and theresidual material can be landfilled or processed into constructionproducts.

Preferably, the methods and apparatuses described herein can be used torecover at least about 50% of each of the wet organic materials and thedry organic materials and the ferrous metals and non-ferrous metals. inthe initial solid waste stream. More preferably, at least about 75% ofeach of the wet organic materials and the dry organic materials and theferrous metals, non-ferrous metals and glass contained in the initialsolid waste stream can be recovered using the methods and apparatusesdescribed herein. Even more preferably, at least about 90% of each ofthe wet organic materials and the dry organic materials and the ferrousmetals, non-ferrous metals and glass contained in the initial solidwaste stream can be recovered using the methods and apparatusesdescribed herein. Most preferably, at least about 95% of each of the wetorganic materials and the dry organic materials and the ferrous metals,non-ferrous metals and glass contained in the initial solid waste streamcan be recovered using the methods and apparatuses described herein.

FIG. 8 illustrates an alternative embodiment in which metal recovery isachieved using a plurality of metal separating devices at a plurality oflocations within the process flow. In FIG. 8, a mixed waste 802 ismetered to a presorting conveyor and sorted in step 804. The presortingstep may be carried out in a similar manner as described above withregard to FIG. 1. In step 806, the remaining mixed waste is comminutedto produce a first size distribution. In step 808, the comminuted wasteis then passed by a magnet to recover a portion of the ferrous metal.Due to burden depth, the magnet used in step 808 is preferably asuspended magnet (e.g. a drum magnet) although other magnets may be usedalone or in combination with a suspended magnet. Drum magnets areadvantageous due to the burden depth prior to size sorting.

Following ferrous recovery, the mixed stream is separated by size (step810). Size separation 810 may be carried out as described with regardsto FIGS. 1-3 above or FIG. 4 below. Size separation produces an overfraction 812 and an under fraction 814. Over fraction 812 is enriched indry organics and under fraction 814 is enriched in wet organics. Underfraction 814 may be further processed in step 816 using magnets torecover a ferrous product 818 and yet further processed in step 120using an eddy current separator to produce a non-ferrous product 122.The remaining stream may be primarily wet organics 822, but may alsoinclude a substantial percentage of non-metal inorganic material. Thewet organics and/or non-metal inorganic materials may be landfilled orfurther processed as described herein.

The over fraction 812 is enriched in dry organics and is furtherfractionated using density separation (step 824) to produce a pluralityof intermediate waste streams. Density separation 824 produces a heavyfraction 826 and a light fraction 828. Heavy fraction 826 is enriched inwet organics and inorganic materials. Heavy fraction 826 may beprocessed in steps 830 and 832 to recover ferrous metals 834 andnon-ferrous metals 836 similar to steps 816 and 820, respectively (andproducing wet organic product 838 that may be processed or discarded.Optionally heavy fraction 826 can be combined with under fraction 814 toperform the ferrous and/or non-ferrous recovery steps. Since the burdendepth in steps 816 and 830 is lower than in step 808, a cross-beltmagnet is typically sufficient to capture the ferrous metals (although asuspended magnet may be used if desired). The non-ferrous recovery istypically carried out using an eddy current separator apparatus orsimilar device.

The light fraction 828 from density separator 824 is enriched in dryorganics. Light fraction 828 may be processed in step 840 using magnetsto recover ferrous metals 842. In one embodiment, a suspended magnet maybe used.

Light fraction 828 may be further process to recover non-ferrous metals846 (step 844). Step 844 may be carried out using one or more eddycurrent separators or devices that perform a similar function. Inaddition to recovery of non-ferrous metals, light fraction 828 may alsobe processed to remove polyvinyl chloride (PVC) plastics and/or recovercertain recyclable plastics such as PETE and/or HDPE 850 to yield a dryorganic product 852.

FIG. 9 illustrates yet another embodiment illustrating a process flowincluding a plurality of metal recovery devices. Unless otherwisespecified, the steps described in FIG. 9 may be carried out in a similarmanner as corresponding steps described in FIGS. 1-4 and 8 above. Withreference to FIG. 9, a solid municipal waste 902 is provided thatincludes a mixture of wet organic material, dry organic material, andinorganic material. In step 904, the mixed municipal waste 902 ismetered to a pre-sorting conveyor where high value items, difficult togrind items, and/or hazardous materials are manually picked from thewaste stream. In step 906, the presorted mixed waste stream 902 iscomminuted to produce a mixed stream 902 having a desired particle sizedistribution. Following comminution and prior to size separation,ferrous metals can be recovered (step 908) using a magnet. In oneembodiment, the magnet may be a suspended magnet (e.g., a drum magnet)although other magnets such as cross-belt magnets may also be employed.

In step 910, the mixed waste stream 902 is fractionated in a first sizeseparation to produce two intermediate waste streams, an under fraction912 (i.e., the fine fraction) and an over fraction 914 (i.e., the coarsefraction). The under fraction 912 may be process to recover additionalferrous metals (step 916). In step 916, the burden depth of the underfraction 912 will typically be lower than comminuted mixed waste 902since a portion of the waste stream is separated in step 910. The lowerburden depth can expose additional ferrous metals that may have passedthrough step 908 without being recovered. Under fraction may be furtherseparated by size in step 918 to produce a fine wet product 920 and acoarse under fraction 922. Fine wet product 920 may be primarily wetorganic product but may also include a small amount of inorganicmaterial. Fine wet product 920 may be processed to remove non-ferrousmetals, digested to extract the caloric value of the organics, orlandfilled as residue.

In step 924, the coarse wet fraction 922 is separated in a first densityseparator to produce a residual inorganic fraction 928 and a mixed lightstream 926 that includes wet organics and dry organics. In step 930, themixed light stream 926 is delivered to a second density separation andseparated into a light stream 932 and a heavy stream 934. Light stream932 includes primarily dry organics 936. Dry organics 936 may becombined with a light fraction 938 from third density separation 940 tobe processed into a dry organic product or processed separately.

The light fraction 934 includes primarily wet organics. Light fraction934 may be process to recover non-ferrous metals (step 942) therebyyielding a wet organic product 944.

The over fraction 914 from first size separation 910 is furtherprocessed in a third density separator (step 940) to produce a lightfraction 938 and a heavy fraction 946. Light fraction 938 includesprimarily dry organics and heavy fraction 946 includes a mixture of wetorganics and inorganic materials. Heavy fraction 946 is furtherprocessed in a fourth density separator (step 948) to produce a heavyfraction 950 and a light fraction 932. Heavy fraction 950 and lightfraction 952 can be further processed separately to recover ferrousmetals (steps 954 and 956, respectively) and to recover non-ferrousmetals (steps 958 and 960, respectively) to yield wet organics 964 andinorganic residue 962. In steps 956 and 954 ferrous metals can berecovered using a cross-belt magnet or other suitable magnet. In steps958 and 960, non-ferrous recovery can be carried out using an eddycurrent separator or other suitable device.

Heavy fraction 950 may also be processed to recover stainless steel(step 966). Recovering stainless steel may be carried out using astainless steel metal detector sorter or other suitable device.

Light fraction 938 from the third density separator 938 is processed toproduce a dry organic product 976. In step 968, ferrous metals can beremoved using a suspended magnet or other suitable magnet. In step 969,non-ferrous metal 970 can be recovered using one or more eddy currentseparators. In some embodiments, light fraction 938 may includesubstantial quantities of non-ferrous materials (e.g., aluminum). Inthese embodiments, two or more eddy current separators placed in seriesmay be used to improve the efficiency of non-ferrous metal recovery. Thenon-ferrous metals 970 may optionally be further processed to remove rediron (step 972). Removing red iron (e.g., copper and brass) can becarried out using a camera sorter and metal detector or other device.

Following recovery of non-ferrous metals, light fraction 938 may also beprocessed to remove polyvinyl chloride (PVC) plastics (step 974) andrecover certain recyclable plastics such as PETE and HDPE. The removalof PVC and recovery of PETE and HDPE can be carried out using one ormore optical sorters. The resulting light fraction 938 (i.e., followingPVC removal) is a dry organic product 976 that may be used to as anorganic fuel as described herein.

III. Systems for Separating Municipal Solid Waste

Referring now to FIG. 4, a system 400 for separating municipal solidwaste is disclosed. In one aspect, the system 400 includes a comminutingdevice 402 configured for comminuting an initial municipal solid wasteto produce a comminuted waste having a first size distribution. A numberof solid waste grinders or shredders available in the marketplace areeither adapted or can be adapted for grinding the initial solid wastestream. For example, Vecoplan, LLC of High Point, N.C. makes a number ofsolid waste shredders that can be incorporated into the system 400described herein.

A typical solid waste grinder or shredder may include one or more shaftsthat include a number of cutting heads that that can cut and/or shredincoming waste materials to a selected size. Wastes ground or shreddedby the grinder or shredder will have a range of particle sizes. Forexample, some objects like shipping pallets or tires will be ground orshredded, but most particles will be relatively large. In contrast,materials like glass, which tends to shatter, and food waste, whichtends to shred, will be quite small.

The product of the solid waste grinder or shredder 402 can be conveyedto a first size separator 404. Suitable examples of size separatorsinclude trommel screens, vibratory screens, finger screens, discscreens, and the like. Preferably, the first size separator 404 is adisc screen. Disc screens are available from Vecoplan, LLC of HighPoint, N.C. A disc screen employs a series of rolling shafts having aseries of attached discs with spaces between the discs that objects canfall through. The rolling of the shafts creates a unique wavelike actionthat agitates the incoming material as it is conveyed forward. Thisagitation releases smaller materials through the screen openings and isaccomplished without vibration or blinding. The disc screen designgreatly reduces the possibility of jamming or seizing during operation.When compared to trommels, vibratory, or finger screens, advantages ofdisc screens include high capacity, small footprint, accurate materialsizing, self-cleaning action and low operational and maintenance costs.

As waste materials are ground or shredded in the comminuting device 402by turning rotors mounted with cutting blades or knives against a rigidblade housing, they then drop through the grinder or shredder to thescreen basket. Materials having a ground size less than a selected size,such as about 16 inches or less, about 14 inches or less, about 12inches or less, about 10 inches less, or preferably about 8 inches orless then drop through a screen where they can be processed by the firstsize separator 404. Objects that are too large to pass through thescreen are typically recirculated repeatedly through the grinder orshredder until they are ground to a size that can pass through thescreen.

Preferably, the comminuted waste from comminuting device 402 is groundor shred to a size of about 16 inches and below, more preferably lessthan about 12 inches and below, and most preferably less than about 8inches and below. The over fraction from the size separator 404preferably has a size distribution with a ratio of small particles tolarge particles of less than about 10 (i.e., the ratio of the uppercut-off to the lower cut-off has a ratio less than about 10), morepreferably, less than about 8, even more preferably less than about 6,and most preferably less than about 4. Preferably, the under fractionfrom the first size separator 404 has a top size cut-off of less thanabout 6 inches, more preferably less than about 5 inches, morepreferably less than about 4 inches, even more preferably less thanabout 3 inches, and most preferably less than about 2 inches.

The system 400 further includes, a first density separation unit 408configured for processing the under fraction 406 from the first sizeseparation unit 404 to produce a first heavy fraction 410 (i.e., a wetorganic product 412) and a first light fraction 414. The system 400further includes a second density separation unit 416 configured forprocessing the light fraction 414 from the first density separation unit408 to produce a light fraction 422 (i.e., a dry organic product) and aheavy fraction 418 (i.e., a wet organic product 420).

Optionally, the system may include a second size separation unit 426disposed between the first size separation unit 404 and the firstdensity separation unit 408. The second size separation unit 426 isconfigured to produce a second over fraction 403 and a second underfraction 428. The second under fraction 428 includes a wet organicproduct 412. The second over fraction 403 is conveyed to the firstdensity separation unit 408.

In one aspect, the second size separation 426 unit is configured toproduce a second over fraction 403 having a second size distributionwith a ratio of large particles to small particles has a sizedistribution with a ratio of less than about 10, more preferably lessthan about 8, even more preferably less than about 6, and mostpreferably less than about 4 and a second under fraction 428 having anupper particle size of less than about 1/10^(th) of the large particlesize of the over fraction, more preferably less than ⅛^(th), even morepreferably less than ⅙^(th), and most preferably less than ¼^(th). Inanother aspect, the second over fraction 403 has a range of particlesizes from about 2″ to about ⅜″. In yet another aspect, the underfraction from the second size separation unit 426 has a size of lessthan about ⅜″ or less than about ½″.

Returning to the over fraction 432 from the first size separation unit404, the system 400 can further include a third density separation unit434 configured for processing the over fraction 432 to produce a heavyfraction 440 and a light fraction 436 (i.e., dry organic product 438),and a fourth density separation unit 442 configured for processing theheavy fraction 440 from the third density separation unit 434 to producea heavy fraction 448 and a light fraction 444 (i.e., a wet organicproduct 446).

The system 400 can optionally include at least one metal separation unit452 that may include one or both of a magnetic separation unit or aneddy current separation unit configured for recovering a ferrous metalfraction 454 or a non-ferrous metal 456 from one or more of the underfraction, the over fraction, or a downstream portion of these fractions.

Referring now to FIGS. 5 and 6, examples of density separation unitsthat are well adapted for separating municipal solid wastes by densityare shown. While the particular examples illustrated in FIGS. 5 and 6may be preferred in some embodiments, other separators can be used.Density separators suitable for use in the present invention include,but are not limited to air separators available from WesteriaFördertechnik GmbH, Ostbevern, Germany. FIG. 5 illustrates a so-calledair drum separator 500. FIG. 6 illustrates a gravity/air separator 600,which is known by one manufacturer as a Windsifter.

The air drum separator 500 illustrated in FIG. 5 includes an inputconveyor 504, a blower 506, a rotating drum 510, an output conveyor 522,a heavy fraction conveyor 518, a light fraction conveyor 526, and an airreturn unit 514. Mixed density wastes 502 are fed in on the inputconveyor 504. As the waste material 502 is fed in, it drops off the endof the conveyor 502 where the wastes 502 encounter a stream of movingair 508 from the blower 506.

The heavy fraction 516 is separated from the mixed waste material 502 byvirtue of being too heavy to be lifted by the airstream 508. The heavyfraction thus falls down in front of the drum 510 and falls on to theheavy fraction conveyor 518. In contrast, the lighter wastes are liftedup by the airstream 508 and carried over the rotating drum 510 andcarried forward either by the air flow 520 or by the conveyor 522. Thelight fraction 524 drops off the end of conveyor 522 and onto the lightfraction conveyor 526. These machines are highly adjustable to alter theweight density separation coefficient, as desired.

The relative density of the heavy fraction 516 and the light fractions524 can be adjusted by controlling the airflow through the air drumseparator 500. The velocity of the airflow and the volume of air passingthrough the drum separator 500 can be controlled either by increasing ordecreasing the velocity of fan 506 or by opening or closing valve 512.In general, opening valve 512 and/or increasing the velocity of the fan506 will carry heavier objects over the drum 510 such that the lightfraction will have a higher average mass. Likewise, closing valve 512 orlowering the velocity of the fan 506 will cause the heavy fraction 516to have a lower average mass and the light fraction 524 will have alower average mass because only the lighter objects will be carried overthe drum 510.

Referring now to FIG. 6, the gravity/air separator 600 includes an inputconveyor 604, a blower 606, an air expansion chamber 622, a heavyfraction conveyor 616, and a first valve 610 and a second valve 612 forcontrolling the volume of air flowing through the gravity/air separator600. Mixed density wastes 602 are fed in on the input conveyor 604. Asthe waste material 602 is fed in, the wastes 602 drop off the end of theconveyor 602 where they encounter a stream of moving air 608 from theblower 606.

The heavy fraction 614 is separated from the mixed waste material 602 byvirtue of being too heavy to be lifted by the airstream 608. The heavyfraction 614 thus falls down on to the heavy fraction conveyor 616. Incontrast, the lighter wastes 620 are lifted up by the airstream 608 andcarried into and out of the expansion chamber 622 by air flow 618.

The relative density of the heavy fraction 614 and the light fractions620 can be adjusted by controlling the airflow through the drumseparator the gravity/air separator 600. The velocity of the airflow andthe volume of air passing through the gravity/air separator 600 can becontrolled either by increasing or decreasing the velocity of fan 606 orby opening or closing the first valve 610 and the second valve 612. Ingeneral, opening valve 610 and/or 612 and/or increasing the velocity ofthe fan 606 will carry heavier objects up into the expansion chamber 622such that the light fraction will have a higher average mass. Likewise,closing valve 610 and/or 612 or lowering the velocity of the fan 606will cause both the heavy fraction 614 and the light fraction 620 tohave a lower average mass because only the lighter objects will becarried up by the air flow 618 into the expansion chamber 620.

Density separators like those illustrated in FIGS. 5 and 6 and the likework best when the ratio between the largest and smallest objects beingfed into the density separator is relatively narrow. Accordingly, it ispreferable that the ratio of the largest to smallest objects that arefed into the density separators in the methods and systems describedherein be about 12 to 1, about 10 to 1, about 8 to 1, or about 6 to 1.Most preferably, the ratio of the largest to smallest objects that arefed into the density separators in the methods and systems describedherein is about 4 to 1 (i.e., where the ratio of the top-cut to thebottom cut are in the foregoing ratios). In one embodiment, the methodsand systems of the present invention are designed to provide wastematerials to the density separators with particles size ratios withinthese approximate ranges.

In yet another alternative embodiment, the fourth density separator 442may be a hammer mill separator. The hammer mill separator may be usedalone or in combination with an air separator. FIG. 7 illustrates anexample hammer mill separator 700 that can be used with the presentinvention. Separator 700 includes a housing assembly 710. Housingassembly 710 can include an inlet 712 for feeding a waste stream intointerior chamber 720 that houses rotating shaft 724. A motor and drivemechanism can be attached to end 734 of shaft 724 for rotating shaft 724at a desired rotation speed.

Housing assembly 710 may also include outlet 714 for discharging a smallparticle sized heavy fraction and a second outlet 716 for discharging alarge particle sized light fraction. Covers 722 a and 722 b can be hingeattached to other components of the housing assembly 710 to provideaccess to interior chamber 720 for cleaning and/or troubleshootingproblems. Housing assembly 710 may also include stands 718 a and 718 bto lift chamber 720 off of a surface and provide access to outlets 714and 716. However, a stand is not required and other configurations mayalso be used.

As mentioned, interior chamber 720 houses rotating shaft 724. Rotatingshaft 724 includes a plurality of hammer shafts that extend generallyperpendicular to the longitudinal axis of the rotating shaft 724.Rotating shaft 724 includes rows of beater blades 730 and breaker bars732. Beater blades 730 include a flattened surface that produces an airflow in chamber 720 that moves parallel to the longitudinal axis ofrotating shaft 724.

Breaker bars 732 are configured to hammer particles flowing in chamber724 and fracture them to produce smaller sized particles. Breaker bars732 have a paddle portion with the paddle angled perpendicular to thedirection or rotation. In other embodiments, the breaker bars 732 can bea hexagonal shaft or other shaped shaft that creates little or nodirectional flow of air through chamber 720.

Hammer mill separator 700 separates particles based on the flowabilityof particles through chamber 720. Particles that are less susceptible tofracturing and more susceptible to the lifting action of the airflow inchamber 724 are forced toward exit 716. Particles that are moresusceptible to comminution and less susceptible to being lifted by airflow are driven toward exit 714.

Interior chamber 720 optionally houses a screen 728 having a particularmesh size. Screen 728 can ensure uniformity in the cut off particle sizeand increase the duration that a particular sized particle remains in apath of the beater blades 730 and breaker bars 732. The screen mesh sizecan be selected to produce any of the distributions described herein.

While many of the methods and systems disclosed herein have beendescribed as including density separation, those skilled in the art willrecognize that in some embodiments, sufficient separation can beachieved without density separation, so long as the waste stream iscomminuted and separated by size to produce intermediate streamsenriched in at least one recoverable material.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

I claim:
 1. A method for recovering recyclable materials from mixedsolid waste stream, comprising: providing a mixed solid waste streamthat includes a mixture of mixed wet organic waste materials, mixed dryorganic waste materials, and mixed inorganic waste materials;fractionating the mixed waste stream by size by performing an upper sizecut and a lower size cut to produce a sized waste stream, the sizedwaste stream having a size range where the ratio of an upper cut-off toa lower cut-off is less than 8; fractionating the sized waste stream bydensity using at least three density separators in parallel and/or inseries to produce at least three separated waste streams including atleast a separated mixed wet organic stream enriched in mixed wetorganics, a separated mixed dry organic stream enriched in mixed dryorganics, and a separated mixed inorganic stream enriched in mixedinorganic materials; and recovering at least one product from each ofthe at least three separated waste streams or a downstream portionthereof.
 2. The method of claim 1, wherein the upper size cut is carriedout using a first screen and the lower size cut is carried out using asecond screen.
 3. The method of claim 1, wherein the ratio of the uppercut-off to the lower cut-off is less than
 6. 4. The method of claim 1,wherein the sized waste stream has an upper cut-off in a range from 8inch to 16 inch.
 5. The method of claim 1, wherein the lower size cutoffis than or equal to 2 inches.
 6. The method of claim 1, wherein at leasttwo of the three density separators are air-drum separators.
 7. Themethod of claim 1, further comprising a fourth density separator that isused in series with at least one of the first, second, and third densityseparators to produce at least a portion of one of the three separatedwaste streams.
 8. The method of claim 1, wherein the three densityseparators are configured to produce the separated wet organic streamfrom materials having a density greater than 12 lbs/ft³ and less than 60lbs/ft³, the separated dry organic stream from materials having adensity less than 12 lbs/ft³, and the separated inorganic stream frommaterials having a density greater than 100 lbs/ft³.
 9. The method ofclaim 1, wherein the mixed dry organic stream is fractionated to producea plurality of dry organic products.
 10. The method of claim 7, whereinthe plurality of dry organic products includes a paper product and aplastic product.
 11. The method of claim 1, wherein the separatedinorganic stream is processed using an eddy current separator to producean aluminum product.
 12. The method of claim 11, further comprisingprocessing the inorganic stream using a magnet to produce a ferrousproduct.
 13. The method of claim 1, further comprising processing thewet organic waste in a digester, wherein the product recovered from thewet organic stream includes a compost product and/or a biogas product.14. The method of claim 1, further comprising processing the separateddry organic stream to produce a dry organic fuel.
 15. The method ofclaim 1, wherein the plurality of density separators produce at least afirst and a second separated dry organic stream enriched in dryorganics.
 16. The method of claim 15, further comprising combining thefirst and second dry organic streams.
 17. The method of claim 16,further comprising recovering the at least one product from the combinedfirst and second separated dry organic streams.
 18. The method of claim1, further comprising shredding the mixed organic waste or a downstreamportion thereof.
 19. The method of claim 1, wherein the shredding isconfigured to produce the upper size cutoff of the mixed solid wastestream.
 20. The method of claim 1, further comprising separatingpolyvinyl chloride from the separated mixed dry organic stream.